The present invention relates to a microwave filter which is particularly suitable for automotive radio communications.
Conventional microwave filter comprises a conductive casing and a plurality of parallel transmission lines each acting as a resonator tuned to a specific frequency in the microwave region. The bandwidth of the filter is determined by the amount of interstage coupling between adjacent transmission lines. For microwave filters in which microwave energy distributed along one transmission line is directly coupled to another through the space between them, the bandwidth is inversely proportional to the spacing between transmission lines. This results in microwave filters having different overall dimensions depending on the different bandwith requirements and is thus disadvantageous for mass production.
Another microwave filter design involves the use of a plurality of shielding members each located between adjacent transmission lines and provided with an opening through which the microwave energy of one transmission line is coupled to another. Although the latter results in microwave filters having a uniform overall size, this involves complicated design procedures.